Make parallel devices optional across all plugins

[SeanNaren]

What does this PR do?

I noticed when running a #6047 using a custom DDPPlugin, that we do not make parallel devices optional, which can be injected at runtime via the accelerator connector. This means we can't enable find_unused_parameters=False or load certain plugins to pass into the trainer.

This PR makes parallel devices optional across all training type plugins.

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[codecov]

Codecov Report

Merging #6051 (63a8952) into master (77f6aa4) will decrease coverage by 0%.
The diff coverage is 100%.

@@ Coverage Diff @@ ## master #6051 +/- ## ====================================== - Coverage 93% 93% -0%  ====================================== Files 160 160 Lines 11400 11406 +6 ====================================== + Hits 10622 10627 +5  - Misses 778 779 +1 

[SeanNaren]

I'll add tests to make sure this works

[tchaton]

Great catch !